1. Field of the Invention
This invention pertains to electrically conductive compositions having utility in electronics. Most particularly, the invention pertains to glass frit and thick film conductive and resistive materials fired in inert or reducing atmospheres. The inventive compositions are used together with or adjacent to materials which are highly reducing.
2. Description of the Related Art
Glass technology originated several thousand years ago. As is typical of an older, more mature technology, there are innumerable compositions designed for many diverse uses. Glass is simply all around us.
In the electronics industry glass plays a crucial role through the desirable features of temperature and humidity resistance, hermeticity, low thermal expansivity, low dielectric constant and high dielectric strength. For the purposes of this disclosure, glass compositions are herein defined to encompass all materials which might in process be reflowed or sintered, including devritifiable or crystalline compositions which might also often be referred to as glass-ceramics or simply as ceramics.
Thick film inks or pastes are well known in the industry, and a number of different glass compositions have been proposed for use therein. Several such compositions are illustrated in U.S. Pat. Nos. 5,043,302, 5,082,804 and 5,093,285 incorporated herein by reference. Commonly preferred glass compositions of the prior art often include borosilicates, due to high temperature characteristics, good thermal shock resistance, good chemical resistance and high moisture resistance typical of this type of glass. The borosilicates are so named because they include both boron and silicon in a silicate structure. Borosilicates may commonly also be referred to as lead borosilicates, aluminum borosilicates or other similar terms which identify other components of the glass composition beyond the silicon and boron.
In previous years, these well known borosilicate glasses performed admirably in many electronic applications, and in fact, these are likely a composition of choice in many, if not most, electronic applications. However, more recent opportunities have begun to reveal some limitations of these materials. Particularly with efforts to reduce the use of precious metals and also gain better electrical conductivity, the industry has begun to utilize more and more copper material to pattern conductive circuits. Copper provides reduced solder leaching and less moisture induced migration than does silver, while sacrificing little in intrinsic conductivity. However, because copper is readily oxidized when heated in an oxidizing atmosphere, processing of copper thick film circuits and materials requires firing in an inert or reducing atmosphere. Typically, dry nitrogen is used in the firing process.
While nitrogen is not in and of itself particularly reducing, the nitrogen firing required for the conductive will also affect the resistor material. This means that associated resistor materials must also be able to be fired in a nitrogen atmosphere. The industry standard ruthenium dioxide resistor material, also invented by the assignee of the present invention, does not fire properly in nitrogen, but is instead adversely affected. In conjunction with copper, other base metal materials are therefore commonly used as resistors. Among these materials are titanium silicide and tin oxide, as for example illustrated in U.S. Pat. Nos. 4,639,391, 4,655,965, 4,698,265, 4,711,803, and 4,720,418 assigned to the assignee of the present invention and incorporated herein by reference.
Titanium silicide has been found to be particularly reducing to surrounding oxide glass compositions when fired in nitrogen. This may lead to a change in component values and characteristics, and even loss of substrate adhesion, where prior art glass compositions are used. Other materials with which a similar problem would likely be encountered would include other silicide materials, as well as many borides and other similar materials of a somewhat reducing nature.